Water-soluble polyvinyl alcohol-coated wax sheet

ABSTRACT

Disclosed is a wax sheet coated with a water-soluble polyvinyl alcohol, said sheet being uniformly perforated. The coated perforated sheet is used to form the integral thermal chamber of a mold or structural part. A method of making the structural part is also provided.

United States Patent Mariano D. Lombardo 7 Lodge Lane, Plrslppany, NJ.07054 789,851

Jan. 8, 1969 Nov. 2, 1971 Inventor Appl. No. Filed Patented WATERSOLUBLEPOLYVINYL ALCOHOL- Field ofSearch 161/109, 112,234; 117/92, 161 UE.98,4; 18/34.1,47 C; 249/1 12-1 14; 264/316. 338

References Cited UNITED STATES PATENTS 3,215,763 11/1965 Buerger3,431,331 3/1969 Pincus et a1 Primary Examiner-Robert F. BurnettAssistant ExaminerLinda M. Carlin Artorney0wen 8L Owen ABSTRACT:Disclosed is a wax sheet coated with a watersoluble polyvinyl alcohol,said sheet being uniformly perforated. The coated perforated sheet isused to form the integral thermal chamber of a mold or structural part.

A method of making the structural part is also provided.

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WATER-SOLUBLE POLYVINYL ALCOHOL-COATED WAX SHEET BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to structuralparts, more particularly plastic molds or dies, and to the method ofmaking them.

In recent years plastic molds or dies have been used in many areas andfor many purposes for which metal molds were formerly used. There areseveral advantages which accrue as a result of using plastic molds. Byusing plastic molds where suitable instead of machined metal,significant savings are achieved in tooling time and costs. They areeasy to repair, and are relatively light in weight and easy to handle.

Heat transfer is a problem with plastic molds and other structural partsbecause plastics are not good heat transfer materials. Good heattransfer characteristics are required in molds because it is sometimesessential that the object being molded be heated in order to speed upthe curing process. This is conventionally done by heating themold. Theheat then transfers from the mold to the object being molded. Sometimesthe part also needs to be cooled, or thermocycled, or maintained atconstant temperature. This can be accomplished only if the mold has goodheat transfer characteristics.

2. The Prior Art In the prior art plastic molds conventionally havecoils molded into the form close to the surface through which heattransfer fluid is circulated. These coils are generally formed frommetal or plastic tubing. Fluid pumped through the coils expedites thetransfer of heat either to or away from the surface of the mold.

A problem which arises with the use of such coils in plastic molds,however, is that in general the coefficient of expansion of the coils isdifferent from that of the plastic in which they are embedded. When suchcoils expand as a result of hot heat transfer fluid being circulatedtherethrough they can conceivably cause cracking of the plastic mold.Conversely, when coolant is circulated through the coils, they canshrink away from the plastic in which they are embedded, diminishing theeffectiveness thereof.

In addition, it is difficult to get uniform heat transfer throughout themold when coils are used.

In many instances of the prior art, electrical heating tapes or blanketsare embedded close to the surface of the mold or structural part. Thismethod is expensive and structurally critical, however, in that itpromotes stresses and cracks on the surface of the mold or otherstructural part.

A new method of making thermally controlled plastic molds or otherthermally controlled structural parts has now been discovered whicheliminates the necessity of embedding heat transfer coils or electricaltapes within the mold, thus allowing less expensive and more efficientmolds to be made.

SUMMARY OF THE INVENTION In its broadest aspect this invention comprisesa thermally controllable structural part which has an internal cavitydefined by generally parallel walls and perimeter walls connecting theparallel walls; a plurality of support columns extending through theinternal cavity, and at least one heat transfer fluid inlet and at leastone heat transfer fluid outlet communicating with the cavity, andthrough which a heat transfer fluid can be introduced into the cavity,circulated and evacuated. Included in the definition of structural partswould be such things as radomes, heated or cooled parts in aircraft,spacecraft and the like having an integrally constructed thermalchamber. Also encompassed within the definition are molds, tools, diesand the like. While not limited thereto, this invention is described interms of a mold for purposes of illustration.

In one embodiment, the mold of this invention comprises a hollow casingof a reinforced, cured, mold-grade resin composition having one sidewhich has an appropriate configuration relative to the object to bemolded, said casing having an internal cavity defined by generallyparallel walls and perimeter walls connecting the parallel walls, aplurality of support columns consisting of a cured, mold-grade resincomposition extending through said cavity between opposite wallsthereof, and at least one heat transfer fluid inlet and at least oneheat transfer fluid outlet communicating with said cavity, and throughwhich a heat transfer fluid (liquid or gas) can be introduced into saidenclosed cavity, circulated and evacuated.

' A preferred mold is one comprising a first layer of a reinforced,-cured, mold-grade resin composition having a surface which has anappropriate configuration relative to the object to be molded, a secondlayer of a reinforced, cured, moldgrade resin composition bonded to saidfirst layer in such a manner as to form a fluid tight enclosed spacebetween the two layers, a plurality of support columns consistingessentially of cured, mold-grade resin composition extending from saidfirst layer to said second layer, and at least one fluid inlet and atleast one fluid outlet communicating with said space through which aheat transfer fluid can be circulated.

A method of making the above described mold comprises the steps of:

a. applying a first layer of a reinforced, curable, mold-grade resincomposition to a pattern,

b. curing the resin composition,

c. applying perforated sheet wax coated with water-soluble polyvinylalcohol to a substantial portion of the back of the first layer,

d. introducing a curable mold-grade resin composition into theperforations in the sheet wax,

e. positioning at least one fluid inlet and at least one fluid outlet inrelation to the coated sheet wax to prevent subsequently applied resinfrom completely enclosing the sheet wax,

f. applying a second layer of a reinforced, curable, moldgrade resincomposition to the coated perforated sheet wax and resin composition inthe perforations to completely enclose the sheet wax and at least aportion of the inlet and outlet within a casing composed of the firstand second layers,

g. curing the resin composition within the perforations of the sheetwax,

h. curing the resin composition of the second layer,

i. heating the coated sheet wax to effect the melting thereof, and

j. removing the melted wax and coating from the enclosure within thefirst and second layers, whereby a fluidtight enclosed space isobtained.

It is an object of the invention to provide a new and useful structuralpart with an integrally constructed thermal chamber.

It is a further object of the invention to provide a method forproducing the structural part.

Other objects and advantages will be apparent from the description tofollow, and from the drawings, in which:

FIG. 1 is a perspective view with parts broken away and parts in crosssection of the mold of the invention;

FIG. 2 is a cross-sectional view of the mold of the invention takengenerally along the line 2-2 of FIG. l;'showing two halves joinedtogether to form a single completed mold;

FIG. 3 is a fragmentary plan view illustrating a part of a mold at oneintermediate stage in production and showing a preferred embodiment ofperforated sheet wax used in the process of the invention;

FIG. 4 is a horizontal sectional view along the line 4-4 of FIG. 2showing the interior arrangement of walls and supporting posts in thecavity of the mold of the invention after the perforated sheet wax hasbeen removed; and

FIG. 5 is a schematic diagram of a preferred heat transfer fluid flowpattern within the internal cavity.

Referring now in more detail to the drawing, and, in particular to FIG.1, the mold of the invention comprises a hollow casing 10 formed of afirst layer ll of a reinforced, cured, mold-grade resin compositionhaving an outer surface which has an appropriate configuration relativeto the object to be molded, bonded to a second layer 12 of asubstantially identical material in such a manner as to form an internalcavity 13. Support columns 14 of cured, mold-grade resin extend from thefirst layer 11 to the second layer 12, and within the cavity 13. A fluidinlet 15 and a fluid outlet 16 extend through the second layer 12 andopen into the cavity 13.

Nonnally, a supporting structure (not shown) is fastened to the backside of the mold.

In normal operation complementary molds are made to form the desiredarticle. Only one-half of a two-part mold has been described above;however, it will be understood that complementary molds can be made inthe same manner. A cross section of two halves is shown in FIG. 2.

To produce a mold according to the method of the invention a pattern 17of the object to be molded is obtained.

Generally the pattern is made of either wood or plastic. The

pattern is then suitably prepared. if the pattern is wood, the surfaceis thoroughly cleaned, and a high-grade wax is applied and rubbed out.Then a parting compound is applied and allowed to dry. Thereafter apaste wax is applied and wiped 011' without rubbing. All of the abovedescribed preparatory steps are conventional to those skilled in the artand do not constitute any part of the invention.

After the pattern is properly prepared, the first step of the method ofthe invention is to apply the first layer 11 of reinforced, curable,mold-grade resin composition to the pattern 17. This can conveniently bedone by first applying a gel coat 18 (a surface coat which is normallyapplied first on the model to render the subsequent layers obscure) ofthe desired resin composition to the surface of the pattern 17, followedby alternate layers of glass fiber fabric and resin 19, until thedesired thickness is reached. Another gel coat 20 frequently is appliedto the back of the last layer of glass fiber fabric. This technique isconventionally known as laminating and is wellknown to those skilled inthe art.

The first layer 11 of reinforced resin, after being built to the desiredthickness, is cured. Depending on the nature of the resin this can bedone at room temperature or at an elevated temperature. If done at roomtemperature curing is accomplished by simply allowing the mold to standundisturbed.

After the first layer 11 is cured, or at least partially cured and is ina tack-free state, at least one perforated sheet 21 (FIG. 3) of wax ofthe desired size and thickness is applied to the back side thereof.Preferably, each sheet 21 is uniformly perforated to such an extent thatthe perforations comprise from about to about 40 percent of the area ofthe sheet, and most desirably about percent. It has been found that abalance of maximum structural strength and optimum heat transferefficiency for the finished mold can be achieved when the perforatedarea is within the above limits. If desired, cutouts 22 can be made inthe wax so that walls can be formed therein for directing flow of fluidas will be explained later herein.

The preferred shape or pattern arrangement of the perforations in thesheet wax is ls-inch round holes on 7/ 16-inch staggered centers. Theshape or pattern arrangement of the perforations may be changed,however, to suit the requirements of the dynamics of the applications.

The sheet of wax is coated with a layer of polyvinyl alcohol, or otherresin which is water soluble or melts at the approximate temperaturethat the wax melts. The resin coating enables the wax to be properlypositioned on the first layer, i.e., the wax can be slid around freelywithout adhering to the resin of the mold, and contaminating thesubsurface upon which it is laid. This could not be done without thepolyvinyl alcohol or other resinous coating. Wax rub-off would result inthe release and subsequent delamination of the reinforced first layerfrom the cylindrical columns cast in the perforations of the sheet 21,as subsequently described. The sheet 21 conform closely to the surfaceto which it is applied and can be readily worked into various shapes.

In the preferred embodiment of the invention substantially the entireback of the first layer 11 is covered by as many sheets 21 of coated waxas may be required. The extent of coverage can be varied as desired;however, as a practical matter, the larger the area of the first layer11 covered, the more efficient heat transfer can be obtained.

After the layer 11 has been covered with the coated perforated sheet orsheets 21, a curable, mold-grade resin composition is introduced intothe perforations in the sheet wax 21. Care should be taken to see thatthe resin fills the perforations entirely and penetrates to the backside of the first layer 1 1. After the perforations are filled, theresin is cured, fonning the columns 14 (FIGS. 1 and 2.)

Next the fluid inlet 15 and the fluid outlet 16 are positioned inrelation to the sheet wax 21 to prevent the subsequently applied secondlayer 12 from completely enclosing the sheet wax. As shown in H0. 1 theinlet 15 and outlet 16 are tubular; however, the shape is not critical.These are generally filled with wax to prevent subsequently appliedresin from blocking the openings.

After the fluid inlet 15 and outlet 16 are positioned, the second layer12 of reinforced, curable, mold-grade resin composition is applied overthe back of the sheet wax 21 and bonded to the aforementioned firstlayer 11, thus forming the casing 10 around the sheet wax 21. The secondlayer 12 can be built up in the same manner as the first layer 11 byapplying alternate layers of resin and glass fiber fabric. The secondlayer 12 is also applied in such a manner that it encloses at least apart of the fluid inlet 15 and outlet 16, leaving the orifices thereofopen. After the second layer 12 is applied, it is cured.

If desired, the cure of the resin in the perforations can be delayeduntil after the second layer 12 is applied, then both can be curedtogether. This results in a strong bond between the columns 14 of resinin the perforations of the sheet wax 21 and the second layer 12.

After all of the resin components are cured, the sheet wax 21 is heatedto a temperature sufficient to cause melting of the wax and polyvinylalcohol or other coating, and the melted wax and coating is thenwithdrawn through the inlet 15 and/or outlet 16 from the cavity 13between the first layer 11 and the second layer 12.

The method by which the sheet wax 21 is melted is immaterial. One methodis to heat the entire assembly to the necessary temperature. When thewax has reached its liquid state a low-pressure air from a hoseconnected to inlet 15 or outlet 16 is applied, ejecting the wax from thechamber 13. After most of the wax is ejected in this manner a finalpurging of the chamber with hot water will completely clean the chamberof any residual polyvinyl alcohol, wax or wax components. Another methodis to inject hot liquid into the inlet 15 or outlet 16 and melt the waxin that manner. Either method is suitable.

When the sheet wax 21 is melted and removed from the internal cavity 13,the support columns 14 are left intact and occupy those spaces analogousto the perforations in the sheet wax 21, providing rigid support for theinternal cavity of the mold. 1f the sheet wax 21 originally had cutouts22 (FIG. 3), then walls 23 F 16. 4) of cured resin would be fonned inthose spaces. The walls 23 serve to direct the flow of fluid within thecavity 13.

Some of the aforegoing procedures described encompass conventional shoptechniques in casting, laminating, spraying etc. The critical aspect ofthis invention, however, involves the usage of the special polyvinylalcohol-coated perforated sheet wax to form the integral thermal chamberin the mold or structural part. The unique and important segment of thisspecification is directly related to the resultant design of the thermalchamber in the structure described herein and also related to theperforated sheet wax which is coated with polyvinyl al' cohol, theutilization of which permits the attainment of the design mentionedbecause of its easy workability.

After the sheet wax 21 is melted out a heat transfer fluid is circulatedthrough the fluid inlet 15 into the internal cavity 13 and removedthrough the outlet 16 to add or remove heat from the mold during use.

The support columns 14 also prevent collapse or cracking of the surfaceof the mold due to normal working pressures.

Almost any resin can be used in the production of the mold of thisinvention as long as it is of mold-grade and capable of withstanding thetemperatures to which it is subjected. Epoxies, polyesters,polyurethanes, phenolics, and silicones have all been sued for toolingpurposes. The preferred resins are heat resistant epoxy resins, mostpreferably those which are cured at room temperature.

There are certain technical considerations which should be observed inthe production of a mold according to this invention. For example, thegel coat l8 of the first layer 11 should be applied as thinly aspossible in order to reduce surface cracking. This coat preferably isallowed to set to a rubbery but tack-free state and then the alternatelayers of glass fiber and resin 19 are applied on top of it. Inapplications where the surface texture of the glass cloth or otherreinforcing agent is tolerable, the laminating resin itself can be usedas a surface coat followed immediately with the glass-laminatingprocedure.

Assuming, however, that the gel coat 18 is applied first, the exposedsurface thereof should be sandblasted or manually sanded. Then at leastfour layers of 6- to IO'ounce glass cloth is applied behind the gel coatwith the aforementioned laminating resin. The number of layers isdependent upon the operational pressure of the heating or cooling fluid.For better heat transfer a minimum thickness of reinforcement is used(within safe limitations).

If desired the reinforced layer 19 behind the gel coat 18 can becomposed of chopped glass fibers interspersed in a resin. This usuallyinvolves the "spray-up of the glass fiber-resin mix onto the gel coat18. If the spray-up method is more practical or desirable for theparticular application, the thickness of the section 19 should desirablybe 1% times the thickness required for glass cloth-layup systems.Whichever method is used the resin/filler ratio must be practicallyconstant to obtain a uniform coefi'icient of thermal expansion betweenthe various sections of the mold.

After the first layer 11 is built up, the perforated sheet wax 21 isapplied to the exposed surface on the back side thereof and is gentlyrolled or pressed into place using the fingers. If the mold contours areintricate the sheet wax 21 can be preheated slightly. lt will thenconform readily to the shape of the mold contour. Caution should beobserved, however, in order that the sheet wax 21 is not heated beyondits melting point. The sheet wax 21 should be trimmed to a boundary atleast one inch from the periphery of the mold. lf zone heating orcooling is required in the design of the mold, two or more sheets 21 canbe applied, unjoined, with a 41-inch spacing between the edges of thesheets extending from side-to-side of the mold. This will facilitateformation of walls forming two or more separate chambers.

A better alternate in forming the fluid circuit is to cut a xinch paththrough the wax 21 where the epoxy or other resin wall is desired. Thepath will subsequently be filled to provide resin barriers or wallswhich control the flow of the heat transfer fluid. There is no limit tothe possible circuit designs which can be achieved by cutting the wallsinto the wax.

FIG. 5 illustrates a preferred center flow pattern through the internalcavity.

The sheet wax 21 is similar to that known as pattem-makers wax which ismodified to meet the requirements of the method of the invention. Thusit must have the desired melting point and be sufficiently pliable to beable to conform to irregular surfaces without substantial elongation ordeformation. A preferred wax for use in the process of the invention isa microcrystalline-modified wax having a melting point of approximately165 F. Other waxes, such as the thermal-stable waxes which are resinmodified, can also be used. These waxes have higher meltingtemperatures. The particular wax used, relative to melting temperaturewill vary depending upon the airing temperature of the mold.

After the fluid inlets l5 and outlets 16 are positioned, the voidsthercaround. if any, can be filled with a resin paste, and the pasteallowed to set.

While the process of this invention has been described in terms of thepreparation of a glass fiber reinforced plastic mold, it can beappreciated that the process is not limited to the production of suchmolds. For example, the process can be applied to cast plastic moldswherein a first layer of plastic is cast and cured and the polyvinylalcohol-coated perforated sheet wax applied to the back of the firstlayer, then the perforations filled and a cast plastic backup coreapplied to the back of the polyvinyl sheet wax and bonded to the firstlayer. In another embodiment a first layer of a reinforced plastic canbe built up followed by the application of the polyvinyl alcohol coatedsheet wax. Then the perforations can be filled and a cast plastic backupcore can be fixed to the back of the sheet wax and bonded to the firstlayer of reinforced plastic. Conversely the first layer can be a castplastic layer and the second layer be a glass-fiber-reinforced plasticbackup core.

In still another embodiment the first layer can consist of a cast metal,conformally plated metal or sprayed metal surface or possibly a ceramicsurface, and the seco'nd layer a reinforced plastic backup core.

Regardless of which of the techniques is used, if an epoxy resin is usedwhich can be cured at room temperature, once the mold is completed theentire mold should be allowed to cure at room temperature for 3 days oralternatively allowed to cure 24 hours followed by a heat cure.

After sufficient cure, the mold is exposed to a temperature sufficientto melt the wax core for removal from its cavity. This can beaccomplished by exposure of the surface of the mold to steam, hot water,radiant heat or heated air to cause a gradual increase in temperature.Finally the cavity should be cleaned of residual wax and polyvinylalcohol with a steam or hot water purge.

The process of this invention can be used in the manufacture of allsorts of plastic molds including those to be used in vacuum-formingprocesses. Vacuum exhaust holes can be drilled through the supportingcolumns 14 and through the first and second layers 11 and 12,respectively, of the mold, thus avoiding leakage of the heat transferfluid to the outside or into the vacuum chamber.

lt is not essential that the first layer 11 and the second layer 12 haveidentical construction. That is, the first layer 11 could be sprayed upwith a mixture of resin and chopped glass fibers or metal fibers and thesecond layer 12 or backup section could be built up withglass-reinforced resin. it should be mentioned, however, that it isnecessary to substantially match the expansion coefficient of the firstlayer 11 and the second layer 12 regardless of which combination isused. Any significant difference in the coefficient of expansion betweenthe first layer 11 and second layer 12 or backup layer will promotedistortion of the layer or cracking of the supporting posts 14 duringthennal cycling of the mold. The final backup structure is builtaccording to the requirements of the application.

In still another embodiment any one of the above constructions can beprepared with two chambers, one behind the other where the one nearestthe surface of the mold is used for heat transfer and the one furthestfrom the surface is used for a vacuum or heat transfer chamber. Vacuumexhaust holes can then be drilled through the supporting columns 14 andthrough the first and second layers Ill and 12, respectively to thesecond cavity which would in turn be connected to a single vacuumexhaust outlet.

The method of this, invention can be applied to the production of anumber of different types of structural parts, and is not limited to theproduction of molds or dies but only to the production of structuralparts which contain an integral heat transfer cavity.

It will be appreciated that variations can be made in the structure andthe method of the invention without departing from the spirit and scopeof the appended claims.

What I claim is:

l. A sheet of wax coated with water-soluble polyvinyl alcohol andsufficiently pliable to conform to irregular surfaces and uniformlyperforated to such an extent that the perforations comprise from about25 to about 40 percent of the area of the sheet.

